Septic shock is the leading cause of death in children worldwide claiming more than 5 million deaths per year. Despite antibiotics and improved supportive care mortality remains unacceptably high in children and adults. We do not yet fully understand the progression of bacterial infection to septic shock nor what determines the severity. Immune suppression as well as exaggerated inflammatory responses can both contribute to poor outcomes. Viral and bacterial co- infection can interact synergistically leading to more mortality. Many studies show that viral infection can disrupt epithelial barriers facilitating pathogen adherence and invasion. Other studies show that viral infection plus endotoxin leads to worse inflammation and mortality. During viral infections, the regulation of subsequent inflammatory responses is not well understood. Many groups have shown that antiviral interferons (IFNs) can worsen inflammatory responses and mortality during viral and bacterial co-infection. How IFNs alter responses to subsequent bacterial challenge leading to more toxicity and lethality is not known. This knowledge will be critical for developing strategies to uncouple the synergy between bacterial and viral co-infections contributing to development of sepsis. The goal of this work is to define the molecular pathways through which antiviral IFNs potentiate subsequent innate antibacterial responses. The hypothesis for this work is that antiviral IFNs reprogram innate responses to TLR ligands during viral-bacterial co-infections. This understanding will provide a good foundation for identifying targets for future therapeutic interventions to reduce the incidence and severity of septic shock. We propose the following specific aims: Specific Aim 1: To determine the mechanism for IFN-induced reprogramming of the TLR signalasome. The hypothesis is that IFNs alter the balance between positive and counter-regulatory negative TLR signaling components leading to a net augmentation of TLR signaling. Specific Aim 2: To determine the role of sialidase activation by IFNs on TLR responsiveness. The hypothesis is that IFN-mediated desialylation of cell surfaces can augment TLR signaling. Specific Aim 3: To determine the role of IFN in the synergy between influenza and TLR ligands in vivo. The hypothesis is that IFNs induce the synergy between influenza and TLR ligands. Our proposed work is novel because it will focus on the central role of antiviral IFNs on signaling through TLRs leading to synergy in co-infection rather than focusing on specific pathogen interaction. We expect that identifying the site at which IFNs modulate the TLR signalasome will provide specific targets for intervention aimed at uncoupling the synergy of co-infection and will allow us to pursue development of therapeutic adjuncts to antibiotic and supportive care in septic shock. PUBLIC HEALTH RELEVANCE This project will focus on the synergistic molecular interactions between antecedent viral infection and subsequent bacterial infection which are known to increase the severity of infections. A good example of which is bacterial pneumonia following influenza which was first noted to contribute to death from influenza as far back as the epidemics of 1957 and 1918. Our work will attempt to explain how viral infection creates vulnerability to bacterial infection and how this co-infection can become so severe. By using experimental models we hope to define critical interactions between our responses to viral and bacterial infection in order to guide the development of therapies aimed at reducing the incidence and severity of such co-infections.